Disc-centrifuge apparatus

ABSTRACT

Disclosed is a disc centrifuge having several straight radial passages leading to a central offtake. When the disc is rotated at high speed, and a gaseous stream is led to its periphery, particles therein are separated. The disc is preferably stationed near the top of an elutriation chamber; use of arrangements comprising a plurality of disc-containing chambers is taught. Separations occur with a minimum of line pressure drop. With means for changing the number of passages used and the disc rotational velocity, a &#39;&#39;&#39;&#39;coarse tuning&#39;&#39;&#39;&#39; versus &#39;&#39;&#39;&#39;fine tuning&#39;&#39;&#39;&#39; effect is conveniently obtained.

United States Patent Imris Mar. 28, 1972 54] DISC-CENTRIFUGE APPARATUS 3,371,782 3/1968 Meyer et a] .209/144 [121 Pal-1 springdaley 333211333 21132? 212i?if3%11111111111111111111...:aa??il [73] Assignee: Westinghouse Electric Corporation, Pittsburgh, Pa. Primary ExaminerTim R. Miles Assistant Examiner-Ralph J. Hill [22] F'led: 1969 Attorney-A. T. Stratton and Z. L. Dermer [211 Appl. No.: 809,780 [57] ABSTRACT Disclosed is a disc centrifuge having several straight radial U.S. CL passages leading to a central omake when the disc is rotated 58 Field of Search ..209/13s 141, 144, at1118 Speed and a 8mm Stream is led its Periphery, P

209/145, 148 ticles therein are separated. The disc is preferably stationed near the top of an elutriation chamber; use of arrangements [56] References Cited comprising a plurality of disc-containing chambers is taught. Separations occur with a minimum of line pressure drop. With UNITED STATES PATENTS means for changing the number of passages used and the disc rotational velocity, a coarse tuning" versus fine tuning ef- 2,276,761 3/1942 Carey ..209 144 f is conveniently obtainm 2,633,930 4/1953 Carter... ..209/144 X 3,089,595 5/1963 Kaiser ..209/l44 2 Claims, 3 Drawing Figures POWER SOURCE miminmze 1912 f 3,651,941

sum 2 [IF 2 SOURCE INVENTOR.

- I PAUL! ms Attorney DISC-CENTRIFUGE APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to apparatus for classifying particulate material, and in particular to apparatus of the kind comprising a disc-shaped centrifuge member having therein a plurality of straight, radially extending passages that communicate with a centrally located offtake. It has reference to apparatus arrangements of high capacity that comprise the plurality of such disc-centrifuge members, and in certain aspects, the invention relates further to methods of operating apparatus of the kind indicated above.

2. Description of the Prior Art The classification of particles, continuously, according to their size is a problem distinct from that of making a detailed size analysis of a sample. Usually the standards of accuracy are not so rigorous, the intervals of size need not be so narrow, there is greater emphasis on speed and capacity, and measurement of the mass distribution according to size is of secondary importance to the size-sorting operation. It is certainly possible, however, with a continuously operating classifier, to take the catch for a prescribed period of time, e.g., 1 hour, and weigh the quantities in each size range, thus obtaining a size analysis.

The most straightforward method of classification is the use of a sequence of sieves. Unless specialized techniques of micro-sieving are used, however sieve procedures are limited for practical considerations to sizes greater than 50 microns. Another technique, used also for particle size analysis, is that of elutriation, where a train of vertical gas currents is used in generally conical channels, each channel having a successively increasing diameter.

Because of the low terminal velocities of very fine particles, gravity elutriation is not satisfactory for some applications. The technique of centrifugal elutriation can then be used. In centrifugal elutriation, the particles entrained in an air stream are carried inwardly in a radial inflow rotor or gaseous vortex. The particle inertial force, rn'v lr, operates to convey the particle outwardly, while the aerodynamic drag force operates to convey the particle inwardly off the vortex or rotor. The latter force is proportional to the diameter of the particle, and the former force is proportional to the cube of the diameter of the particle. Hence, for smaller particles, the inertial force tends to become smaller than the drag force, and the particles smaller than a critical diameter will be carried inwardly. Hence, the rotor wheel or vortex acts as a sieve, separating particles above a certain size from those below this size. By using a number of stages that operate with different rotational speeds or different diameters, the critical particle diameter (for passage) may be made successively smaller for each succeeding stage.

The rather simple concept using a rotor wheel has not been exploited to any great degree in the field of particle-classification technology. Perhaps this is due to the fact that most classification operations are concerned with larger particles, where sieving or elutriation methods can be used.

Those skilled in the art are aware of the Bahco analyzer, a centrifugal classifier developed in Sweden. The dust is introduced from an annular slot feeder near the periphery of a horizontal rotating disc. Air flows radially inward, with induced swirl, over the face of the disc, tending to carry the particles with it; however, the particles above a critical diameter will not be drawn in with the flow, and are thrown out instead. By varying the air flow or the rotational velocity, the cut-off diameter can be changed. From a recent book, by Terrence Allen titled Particle Size Measurement (Chapman and Hall, (London 1968)) comes the information that the Bahco analyzer uses a sample of about 20 grams, which can then be graded in the size range extending from 5 to 100 microns.

Also known is the Dietert centrifugal particle classifier, in which a spiral inflow air current is set up, as in a cyclone. Here again, larger particles are thrown outward, and a cut-off particle size is obtained which depends on the air flow rate. With this apparatus it takes about 2 hours to separate a small sample, such as 10 grams, into eight fractions. See R. D. Cadle, Particle Size Determination, lnterscience, New York 1955, page 231.

From the above-mentioned book of Allen, there is also known a centrifugal elutriator developed at the British Coal Utilization Research Association. It consists of two parallel circular discs of equal radius, mounted on a motor-driven spindle. Dust-laden air enters near the center of the outer disc and flows radially outward over its surface, and then doubles back between the disc. Larger particles will not be able to enter with the How going in between the discs, on account of the strong swirl and the outwardly directed inertial force on the particles. In the rotor assembly of this apparatus, there are no passages or vanes.

In Kaiser U.S. Pat. No. 3,089,595, there is taught an apparatus that uses inward flow in the space between a flat disc and a disc of curved profile, with the axial space increasing from tip to the hub, and with zig-zag vanes or channels being placed between the discs in the flow space. Dust-laden air is forced to flow through the space between the discs, radially inward, while the disc assembly is in rotation, thereby effecting a separation of the larger particles. The teachings of the patent are such as to imply that the use of zig-zag passages, which is costly to implement, is essential to the achievement of desirable results, and the patent is silent with respect to any of the advantages that are to be obtained in equipment of this kind when it is caused to contain or make use of the particular inventive features that are taught and claimed hereinbelow.

Reference may also be had to l-Iebb U.S. Pat. No. 2,616,563 and to Carey U.S. Pat. No. 2,276,761 both of which disclose the use of a disc centrifuge containing a bladed rotor, rather than the simple drilled rotor that is used in the apparatus of the present invention and is not only simpler in construction but also tends to avoid the secondary eddies, flows and swirls that occur between the blades on a bladed rotor.

In the prior art, there has not to the applicants knowledge been provided an apparatus that, while working with powers or other materials that are relatively fine, such as microns and under, will conveniently serve to classify such materials into several various fractions of desired range of particle size.

Even more noticeably, the prior art has not afforded to persons dealing with U0 powder or other finely divided material with similar properties a convenient and effective way of classifying them into appropriate size fractions. UO powder exhibits a strong tendency to agglomerate. In other words, it is very surface-active, comprising particles of high electrostatic charge or polarization. Even when suspended in an elutriator and subjected to the action of a resonator to disperse agglomerates, the U0 powder will reagglomerate (because of high interparticle Van der Waal forces) once that it leaves the immediate vicinity of the resonator. Workers with such powder have noticed that similar properties are sometimes obtained with size fractions that are nominally different, and dissimilar properties are sometimes obtained with size fractions nominally the same. The art has lacked a way of performing on such a finely divided, agglomeration-prone powder an appropriate size classification that will yield fractions or cuts with dependable performance characteristics as respects the further processing thereof into fuel elements.

BRIEF SUMMARY OF THE INVENTION I The present invention is generally applicable to separations top of an elutriation chamber; use of arrangements comprising a plurality of such disc-containing chambers is taught for the purpose of obtaining a plurality of fractions or cuts of size range as narrow as might be desired. The device affords separations with a minimum of line pressure drop. To that end, the passageways in the disc are 100 times the particle diameter, min. With means for changing the number of passages used and the disc rotational velocity, adjustment of the apparatus in a manner corresponding to the difference between coarse tuning and fine tuning is conveniently obtained. The combination of the elutriation chamber and disc centrifuge, as herein taught, is of particular usefulness in classifying agglomeration-prone sub-sieve-size or otherwise finely divided materials like U powder.

BRIEF DESCRIPTION OF THE DRAWINGS a complete understanding of the invention may be obtained from the foregoing and following description thereof, taken together with the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating the apparatus of the present invention;

FIG. 2 is a view taken on the line IIII, FIG. 1; and

FIG. 3 is a schematic diagram showing the use in series of a plurality of units such as that illustrated in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The apparatus of the invention is illustrated in FIGS. 1 and 2. It comprises a gas line 2 that contains a flow meter 4. Mounted on a table 6, there is a particulate-matter feed means 8 that comprises a hopper l0 and a line 12 leading from the bottom thereof into the gas line 2 and containing a screw feeder 14 or other suitable means for regulating the flow of particulate material into the gas line 2. At 16, the line 2 is joined with the inlet line 18 that leads to the bottom of the elutriation chamber 20. As seen in FIG. 1, the elutriation chamber 20 is joined to the table 6 by suitable means, such as screw threads 22, and its general shape is that of an inverted cone or frustum of a cone, the elutriation chamber 20 having a bottom portion 24 of relatively small horizontal cross section and, in the vicinity of its top, a portion 26 of enlarged horizontal cross section. In the table 6 there is an opening 28, through which there is passed a spindle-like member 30 that is journaled within suitable bearings 32 and has at its lower end a disc 34 that comprises an outer portion 36 and an inner ring 38.

As best seen in FIG. 2, which comprises a central horizontal cross section of the disc portion 34 of the spindle-like member 30, the outer portion 36 thereof has therein a plurality of straight, radially extending passageways 40. These communicate with a central off-take 42 comprising a hollow central portion of the spindle-like member 30 (see FIG. 1). FIG. 2 also shows that the inner ring 38 has, in its central portion a plurality of segments 44-54 that are of differing widths and are so located that by turning the inner ring 38 to an appropriate angular position with respect to the portion 36, one or more of the passageways 40, as desired, may be blocked off. The inner ring 38 fits snugly against the outer portion 36, being secured thereto by suitable means (not shown). To be somewhat more particular, if the inner ring 38 is rotated from the position shown in FIG. 2 counterclockwise with respect to the portion 36, it will be seen that the segment 54 will block off the one passageway 40a of the passageways 40, but none of the other ones of the passageways 40 will be blocked off, because the segment 54 is the largest one of the segments 44-54. Similarly, if the ring 38 is turned 10 clockwise with respect to the portion 36 from the position shown in FIG. 2, all six of the passageways 40 will be blocked off, and if it is turned 30 clockwise with respect to the portion 36 from the position shown in FIG. 2, four of the passageways will be blocked 0E and two will remain open. This provides a means for changing the number of passageways 40 in the disc centrifuge that are open to the passage of gas therethrough,

for a purpose that will be discussed in greater detail hereinbelow. It is to be understood that the means for securing the inner ring 38 to the outer portion 36 of the disc centrifuge should be such as not to throw the disc 34 out of balance, as it must be rotated at high speed. Those skilled in the art will perceive other and equivalent ways of changing the number of the operating passageways 40, for the purposes discussed hereinbelow.

Referring now again to FIG. 1, motor 66 is illustrated schematically to include rotor 56 operatively associated with the spindle 30, and a stator 64 powered by means of power drawn from a source 58 through lines 60 containing a speed regulator 62. Lines 68 lead to a speed meter 70.

The upper end of the spindle 30 communicates through a seal 72 with an off-take line 74. In the line 74, there may be placed a suitable joint 76, similar to the joint 16, by means of which the equipment described above may be connected in series to another similar unit.

FIG. 3 illustrates an arrangement comprising a plurality of interconnected units of the kind described above. There is illustrated a line 78, corresponding to the line 18 of FIG. 1, leading to the vicinity of the bottom of an elutriation chamber that is connected to a table 82. Again, the spindle member 84 corresponds to the spindle member 30 of the unit shown in FIG. 1, and the motor 86 and the disc 88 correspond to the motor 66 and disc 34 of FIG. 1. For simplicity, the seal 72 of FIG. 1 is not shown, and the off-take line 90 of FIG. 3 corresponds to the off-take line 74 of FIG. 1. The foregoing items comprise a first unit 92, which, as shown, is connected in series with similar units 94 and 96. The unit 94 has an elutriation chamber 98 and a disc 100. The unit 96 has an elutriation chamber 102 and a disc 104.

Near the bottom of the elutriation chamber 80, there is a solids-removal line 106 that contains a solids-conveying means such as a screw feeder 108, operatively connected to a motor 110. The line 106 communicates with a hopper 112 that corresponds to the hopper 10 that is shown in FIG. 1. Also shown in FIG. 3 is a separate table 114, upon which the hopper 112 is mounted, and passing through the table 114 is a line 116 that contains a suitable feed means 118, leading to the line 120, to which gas is fed from a source 122 through a metering means 124. The line 120 leads to another unit 126, similar to the units 94 and 96 described above. As will be discussed below in greater detail, an arrangement of the kind shown in FIG. 3 makes possible the relatively rapid and convenient classification of a powder material, even one that exhibits a strong tendency to agglomeration, into a number of size fractions that, as will be shown, may readily be made as broad or narrow as may be desired to suit the particular requirements of the user of the apparatus.

OPERATION Referring to FIG. 1, after the equipment is assembled, with a suitable quantity of the particulate material to be classified being placed in the hopper 10, gas flow is started through the line 2, and the feed means 14 is activated to cause the particulate material to enter the line 18 and be conveyed by it into the bottom portion therein of the elutriation chamber 20. The motor 66 is started at the same time, and it is operated to cause the disc 34 to be revolved at high speed. The dimensions of the disc 34 and its rotational velocity are to be selected appropriately in accordance with the size of particle that the unit is intended to permit to pass. In most instances, however, in the operation of the apparatus of the invention, there will be used a rotational speed of the spindle member 30 of 200 to 200,000 revolutions per minute, and the disc 34 will have a diameter of 200 millimeters to 0.2 millimeters, with the higher rotational velocities being used with the discs of smaller diameter. The disc centrifuge then operates as a filter member upon the particles that rise to the upper portion 26 of the elutriation chamber 20, since an individual particle will have imposed upon it, on the one hand, the centripetal force of aerodynamic drag caused by the passage of gas inwardly through the passageways 40 of the disc 34, and on the other hand, the centrifugal force caused by rotation of the disc 34. It is to be understood that the elutriation chamber works to cause only the smaller, lighter and less dense ones of the particles entering the elutriation chamber to rise to the portion 26 thereof. Because of the frustoconical shape of the elutriation chamber 20, the upward velocity of the gases therein is at maximum in the vicinity of its bottom 24, decreasing as the upper portion 26 of the elutriator is approached. A larger or heavier particle requires a greater upward gas velocity to buoy it up, so the elutriator 20 causes the heavier ones of the incoming particles entering it to remain near its bottom.

The velocity of the gas inwardly in the passageways 40 must, of course, be chosen appropriately with due regard to the critical particle diameter and other operating parameters, and this may be done with the use of the equation u 1817 where r u* is the gas velocity in the passageways 40, cm./sec.

a is the particle diameter, cm. is the particle density, g./cc. is the radius of the disc, cm.

is the rotational velocity, radians/sec.

is the Cunningham correction factor, dimensionless. 1; is the viscosity of the gaseous medium used, poises. Or-

dinarily, values of u* will be used that are on the order of 0.1 to 5 meters per second. Lower values of gas velocity tend to lend greater sensitivity to the separation being effected.

In the operation of the apparatus, initially the gas velocity in the line 2 is kept low and the rotational velocity of the spindle 30 in disc 34 is preferably as high as possible. Also, the means for selecting the number of the passageways 450 that are in use (i.e., that shown in H6. 2, or its equivalent) is so manipulated as to utilize the greatest possible number of the passageways 40 and thereby minimize the flow rate therethrough. At the outset, then, only the very smallest ones of the particles entering the elutriation chamber 20 are permitted to pass into the output line 74.

After an appropriate period of time, the speed of rotation of the disc 34 may be decreased to a lower value. Before decreasing the speed, the disc would have held back all particles above a given size. After decreasing the speed, the disc now holds back all particles above a slightly larger size; in this way, it is discharging particles in a narrow size range, which may be a useful feature. Means would be provided downstream from member 74 to separate the particles from the air stream. Further fine adjustments may be made as necessary or desired. Alternatively, if no especially narrow cuts of material to be classified are required, the rotational velocity of the disc 34 may be maintained constant until the time comes that it is desired to make a larger or coarse adjustment, such as may be done by decreasing the number of the passageways 40 that are in use, thereby increasing the inwardly directed gas velocity in the passageways 40 remaining in use. Desirably, means are provided that permit this to be done without stopping the classification process and disassembling the apparatus.

The apparatus shown in F IG. 3 is operated in a manner that will readily be understood from what has been said above. With additional units working, a greater quantity of material can be suitably classified in a given period of time. It is necessary, of course, that the disc member 104 be rotated at a higher rate of speed than the disc member 100, and the disc member 100 at a higher rate of speed than the disc member 88, provided that the number of passageways being used in each is the same and the units are otherwise identical. What is really essential, of course, is that each succeeding unit, as one passes downstream, be operated under such conditions as to pass only particles that are smaller than those passed by the unit immediately preceding. Thus, although in the drawing,

FIG. 3, the units are shown as being of the same size, those skilled in the art will readily appreciate that in many instances it will be preferable to have the downstream ones comprise copies on a smaller scale of the upstream units preceding them. The apparatus of FIG. 3 shows also how the coarse material separated in the bottom of the elutriation chamber may be conveyed by the line 106 to the hopper 112, so that it may thereafter be processed by the unit 126. Unit 126 may be the first one of a second train of units, corresponding to the units 92, 94 and 96. Additional trains can be added, as desired. The elutriators 80, 98 and 102 may be provided with bottomsremoval means, if desired.

Apparatus of the kind described above is especially useful for the classifying of finely divided particulate material under microns maximum dimension, that has a strong tendency to agglomerate. Uranium dioxide powder is an example of such material. in efforts to develop fuel elements of superior properties for use in nuclear reactors, it has been learned that in at least some instances, there is a correlation between the size of particle used and the performance characteristics. Particles of a certain size range are frequently found detrimental to the properties of the final fuel elements. Nevertheless, the development of this art has hitherto not been able to make rapid progress, because of the lack of a satisfactory means for classifying this hard-to-handle powder on anything but a very small scale. In particular, elutriation has been tried with the result that there was obtained from the top of the elutriator a mixture of particles of light density but of considerable variation in size. It is known that the larger ones of these particles are rather loosely agglomerated by electrostatic or Van der Waals forces, and use of a resonator in the elutriation chamber has shown that although the resonator provides, in its immediate vicinity, sufficient energy to break up the agglomerated particles, there is a strong tendency for them to recombine before any separation can be effected. With the apparatus of the invention, however, the fragmentation and separation of the larger ones of the light particles takes place almost simultaneously, with a result that the desired separation is obtained.

EXAMPLE I As an example of the practice of the invention, there is performed a separation using the apparatus of FIG. 1. There was used a rotor of 75 mm. radius, containing six passageways 40, each having a diameter of 8 mm. The central space 42 in the rotor had a diameter of 24 mm. The rotor spindle was driven by means of an asynchronous electric motor capable of 12,000 revolutions per minute. The elutriation chamber was 24 mm. in diameter in the vicinity of its bottom, 21 cm. in diameter at its top, and sloped at an angle of 60 with respect to the horizontal in its sides. As the gas passing through the elutriation chamber and rotor and into the exit line 74, there is used dry air, the flow rate thereof being such that the gas flow rate inwardly in the passageways 40 is 1 meter per second. The pressure difierential that is required to cause this flow is remarkably low, being on the order of 50 to 75 mm. of water. The powder used is a uranium dioxide powder prepared by the ammonium diuranate process, ranging in particle size from 0.2 to microns, with a maximum at 7 microns (12 weight percent). Of such powder, there is used a sample of 10 milligrams. The rotor is accelerated to 6,000 rpm, stroboscopically determined. The sample is fed into the air stream, being then suitably elutriated in the elutriation chamber, with the finer ones of the particles reaching the vicinity of the rotor, and with the particles of less than 5 microns maximum dimension passing through the passageways 40 and thence into the exit line 74. It is possible to monitor the size of particle being passed in the exit line 74 by inserting momentarily therein a microscope slide lightly coated with silicone oil so as to catch thereon a few of the particles in the gas stream contained in that line. This makes it possible to compare, using an electron microscope or, if the particles are large enough, an optical microscope, the particles caught on the oil film with others of known size. After the equipment has been so operated for a period of about 12 to 15 minutes, all of the minus micron material will have passed through the passageways 40. The rotor speed is then decreased to successively lower values, being maintained at each new value for a minute or two, with the size of particle in the line 74 being in each case determined as indicated above, Further trials were done with different gas velocities in the passageways 40, and the outcome of this work is indicated in the following Table.

TABLE Influence of Gas Velocity and Rotor Speed on Critical Particle Diameter Critical Particle Further work was done, using a larger quantity of material, namely, grams of the same particulate material, uranium dioxide produced by the ammonium diuranate process. The equipment used was the same as in Example 1. Cuts of While 1 have shown and described herein certain embodi- 5 ments of my invention, I intend to cover as well any change or modification therein which may be made without departing from its spirit and scope,

I claim as my invention: 1. Apparatus for classifying particles comprising hollow 10 housing means having a disc member therein,

said disc member having therein a plurality of radially extending passageways communicating with a central otftake and with the exterior periphery of said disc member, said disc member having a diameter not greater than about 200 millimeters,

means for establishing a higher gas pressure at said exterior periphery of said disc member than at said central offtake,

means for causing said disc member to be rotated at a high speed in the range of 200 to 200,000 revolutions per minute,

means for introducing particles to the exterior periphery of said disc member and for introducing a carrier gas into said housing means, and

means for altering the number of said passageways in said disc member through which gas passes.

2. Apparatus as defined in claim 1, characterized in that:

the interior of said housing means forms an elutriation chamber of generally inverted frustoconical shape, and in that said disc member is located in the vicinity of the top of said elutriation chamber, said central off-take communicating with a conduit leading to the exterior of said elutriation chamber. 

1. Apparatus for classifying particles comprising hollow housing means having a disc member therein, said disc member having therein a plurality of radially extending passageways communicating with a central off-take and with the exterior periphery of said disc member, said disc member having a diameter not greater than about 200 millimeters, means for establishing a higher gas pressure at said exterior periphery of said disc member than at said central off-take, means for causing said disc member to be rotated at a high speed in the range of 200 to 200,000 revolutions per minute, means for introducing particles to the exterior periphery of said disc member and for introducing a carrier gas into said housing means, and means for altering the number of said passageways in said disc member through which gas passes.
 2. Apparatus as defined in claim 1, characterized in that: the interior of said housing means forms an elutriation chamber of generally inverted frustroconical shape, and in that said disc member is located in the vicinity of the top of said elutriation chamber, said central off-take communicating with a conduit leading to the exterior of said elutriation chamber. 